1. Field of the Invention
This invention relates to a method of producing staple or powders of certain aluminum alloys which contain transition metals using rapid solidification processing and to their subsequent consolidation and heat treatment to have desirable mechanical properties. The invention also relates to preferred Al-Fe compositions made by this method.
2. Description of the Prior Art
Rapid solidification processing (RSP) techniques offer outstanding prospects for the creation of new, cost-effective engineering materials which may have physical properties superior to those otherwise available (see Proceedings, Int. Conf. on Rapid Solidification Processing; Reston, Va., 1977; Claitor's Publishing Division, Baton Rouge). Depending on alloy composition, RSP techniques can be used to alter the structure and microstructure of the alloys compared to that achieved by ordinary production processes; the high cooling rates typical of high volume RSP processes (.about.10.sup.5 -10.sup.7 .degree.C./sec) can produce metastable phases and prevent or markedly reduce the compositional segregation and can occur during slower solidification.
In particular, RSP can be used to produce metastable extended solid solutions wherein a large excess of a solute element can be retained uniformly throughout the host element or alloy. Upon suitable heat treatment, a fine dispersion of particles of the equilibrium intermetallic phase within the host matrix can be produced. The potential for using this approach to produce unusual dispersion-hardened aluminum alloys having desirable mechanical properties has long been recognized (see the review of T. R. Anantharaman et al, Trans. Ind. Inst. of Metals; Vol 30, December 1977, pp. 423-448).
A wide variety of RSP techniques amenable to commercial utilization are known. One sub-category of these is known as melt spin chill casting (for examples see S. Kavesh, pp. 165-187, Proceedings, Int. Conf. on Rapid Solidification Processing, 1977, and U.S. Pat. No. 4,142,571, Narasimhan) which is especially attractive since it produces a ribbon or sheet (both herein defined as a ribbon) at a high production rate and at low cost: further, the product is uniform in that all parts of the product experience a relatively uniform cooling rate.
It is recognized by those skilled in the art that a wide variety of related RSP techniques can be used to produce ribbons or sheets from the melt.
The generic term "melt spin chill casting" is used here in its most general sense to include all RSP techniques in which the molten metal is brought into contact with a rapidly moving solid substrate of high thermal conductivity so as to form a ribbon or sheet having an average thickness of about 25 to 100 microns, the ribbon or sheet having been subjected to a cooling rate of about 10.sup.5 to 10.sup.7 .degree.C./sec. Thus, the term melt spin chill casting is used to include processes such as "melt extraction" and two substrate techniques such as "twin-roll quenching" (see H. A. Davies in Rapidly Quenched Metals III, Vol. 1, The Metals Society, London, 1978, pp. 1-21).
Since RSP powders would be highly useful to ease subsequent consolidation, several approaches to making such a product have been developed. Generally, atomization of the liquid is utilized, followed by various cooling procedures. A limitation of such RSP powder processes is that a range of liquid droplet sizes is produced and the droplets of different sizes then experience a range of cooling rates, leading to a non-uniform product. Further, the larger droplets may experience low cooling rates which do not produce the desired effect. Screening so as to use only the finest particles leads to a lowered yield and a less economic process. One process which has been extensively studied for aluminum is impinging the liquid droplets formed by atomization onto a solid quench surface. However, non-uniformity was observed in as-quenched alloy made in this manner (see I. G. Palmer, R. E. Lewis and D. D. Crooks in Proceedings, Second Int. Conf. on Rapid Solidification Processing, 1980) as well as in the microstructures of alloys made by consolidating such material (see T. H. Sanders, J. W. Johnson and E. E. Underwood in Proceedings, Second Int. Conf. on Rapid Solidification Processing, 1980).
While a wide variety of elements are potentially useful to produce dispersion hardening in RSP aluminum alloys, in particular the common transition metal elements, iron is an especially attractive additive in part because of its low cost. The Al-Fe system has been extensively studied; in particular, an alloy containing 8 wt% Fe has been shown to have an ultimate tensile fracture strength of approximately 33,000 psi at 350.degree. C. (see C. M. Adam & R. G. Bourdeau in Proceedings, Second Int. Conf. on Rapid Solidification Processing, 1980), significantly higher than that exhibited by conventional aluminum alloys at this temperature.
It appears that there is the need for a method to produce RSP powders of aluminum containing high levels of transition metals which are more uniform than those now available. Further, the inclusion of larger amounts of the transition metals than can now be done without deleterious effects in combination with the improved uniformity can lead to the achieving of properties, in particular high tensile fracture strength at elevated temperatures, better than that which have so far been achieved.